The aerospace industry has been dependent for many years on constant-speed-drives (CSDs), to provide constant-speed input to aircraft generators when the engine speeds vary over a 2:1, or more, speed range. While these drives have been satisfactory, their development has been protracted, in view of the difficulty of compacting sophisticated hydraulic and highly stressed mechanical components into small volumes. Thus these CSDs are generally costly, sophisticated, and more particularly, unsuitable for the large electric generators anticipated to be used in future "all-electric" airplanes.
A variety of differential and variable-ratio transmissions and drives utilized in conjunction with electric machines, are known in the prior art. Such drives are typified by those disclosed in U.S. Pat. Nos. 2,810,844 to Morrill; 3,032,696 to Payne et al; 2,077,768 to Perry; 2,990,503 to Clark; and 2,153,252 to Hansdorf.
In the '844 patent, a constant speed drive type system is disclosed which uses a differential planetary gear arrangement to control the frequency of a dynamo/electric machine, which derives a selected frequency, different from the base frequency of the machine. The '696 patent discloses an engine driven generator/motor that includes a variable-ratio drive interposed between the engine and generator. The drive system includes bi-directional (overrun) clutches to prevent direct drive of the transmission in the generator mode, and gear-reduction in the motor (start) mode. In addition, a current sensing control (current-transformer) is used to monitor and control the power taken by the motor/generator.
A synchronous drive system is disclosed in the '768 patent, wherein a frequency changer is driven at a variable speed, and rotary induction machines are electrically "locked in step" with the frequency generator. In this particular system, the machines operate at the speed of the frequency generator. The '503 patent, by contrast, describes a two directional power flow system that utilizes multiple devices such as a vari-drive, directional clutches, synchronous motor/generator, etc. In this system, the synchronous machine is automatically controlled when in a driving mode of operation, such that the ratio change in the drive gears is adjusted in dependence upon the torque and power of the driving motor, as ascertained by the magnitude of the current drawn by the machine. The system operates to maintain the magnitude of the current at a predetermined level.
Finally, a vari-drive (toroidal-type) transmission is shown in the '252 patent, driving an alternating current machine. The transmission allows for the supplying of power at various selective amounts from a constant speed motor, and provides a means for varying the proportionate actual speeds of rotating field and armature members of an electric machine.
All of the aforementioned prior art patents are directed to power systems quite different from the all electric constant speed/variable speed drive-generator assembly, as described by the present invention.
One power generation system which provides dual-range power over varying engine speed ranges is disclosed in co-pending U.S. patent application Ser. No. 235,354, for "Dual-Range Differential-Drive For AC And Induction Generators," filed Feb. 15, 1981 and assigned to the assignee herein. The application discloses a drive system which utilizes "straight" toroidal or "differential" toroidal drives to provide input speeds to synchronous or induction primary power generators, such that constant power is provided over a preselected speed range of the engines, and power proportional to speed is provided at speeds below the preselected speed range. Such systems do not, however, meet the objective, in an "all-electric" airplane, of being essentially "electrical" in their mode of operation.
The disclosures of all U.S. patents and co-pending patent applications referenced herein are hereby incorporated by reference.
It is a primary object then, of the present invention, to provide a compact, electric drive/generator assembly which operates in a constant-power and variable-power mode.
It is another object of the present invention to provide a power generation assembly, particularly for aircraft, that enables an optimal sizing of the drive and generator, even though they operate over a wide speed range, as is incident-upon operating the airplane through its flight envelope.
It is yet another object of the present invention to provide an electric constant-speed/variable-speed drive/generator which is particularly well adapted to modular construction techniques.
It is another object of the present invention to provide a compact electric/electronic alternative to conventional CSDs for operation in the somewhat hostile environment of aircraft power plants, where hydraulic fluids create prospective fire hazards.
These and other objects of the present invention will become more readily apparent after a full consideration of the following description of the instant invention, and the several advantages thereof enumerated herein.